1. Field of the Invention
The present invention relates to a cylindrical ceramic heater formed by covering a core material with a ceramic sheet.
2. Description of the Related Art
Conventionally, in a ceramic heater which is formed by covering a core material with a ceramic sheet, a heating element is embedded in the ceramic sheet. Concurrently, a lead portion connected to a rear end side of the heating element is also embedded in the ceramic sheet. The lead portion is connected to an electrode pad formed on the outer peripheral side in the cylindrical ceramic sheet through a via conductor in a through hole formed in the ceramic sheet, thereby realizing electrical conduction to the heating element (refer to FIG. 2 of JP-A-2003-317907, among others).
3. Problems to be Solved by the Invention
Although a through hole must be formed in advance in the ceramic sheet to realize electrical conduction to the heating element, in terms of the structure in which the core member is covered with the ceramic sheet, a problem arises in that stress which enlarges the through hole in the circumferential direction occurs while covering the core member with the ceramic sheet. Consequently, a crack is likely to occur in the through hole and its periphery.
The reason is as follows.
The through hole is formed as the ceramic sheet is punched from its obverse surface toward its reverse surface by a processing tool (punch). At this time, considering the inner peripheral surface of the through hole, the obverse surface side of the ceramic sheet is formed into a smooth planar surface conforming to the surface of the tool. However, the reverse surface side is pulled by the tool and can fall off. Thus, there are cases where the inner peripheral surface of the through hole ceases to be smooth.
As for the via conductor which is embedded in the through hole in such a state, since the reverse surface side of the ceramic sheet at the through hole is not smooth, the adhesive force between the via conductor and the inner peripheral surface of the through hole becomes weaker than on the obverse surface side of the ceramic sheet.
Then, if a core member is covered with this ceramic sheet with its reverse surface side set as an outer surface, stress is applied in a direction in which the inner peripheral surface of the through hole and the via conductor are stretched in the circumferential direction. However, at that time, since the adhesive force on that reverse surface side is weak, the via conductor is likely to exfoliate from the inner peripheral surface of the through hole. This in turn induces generation of a crack in the through hole and its periphery.